The present invention is related to Overfire Air systems for reducing nitrogen oxide emissions in combustion systems. More specifically, the present invention provides a step diffuser at the Overfire Air injector outlet end to enhance mixing of air with flue gas to improve Overfire Air system performance.
One of the major problems in modern industrial society is the production of air pollution by a variety of combustion systems, such as boilers, furnaces, engines, incinerators and other combustion sources. One of the oldest recognized air pollution problems is the emission of oxides of nitrogen (NOx). In modern boilers and furnaces, NOx emissions can be eliminated or at least greatly reduced by the use of overfire air (OFA) technology. In this technology, most of the combustion air goes into the combustion chamber together with the fuel, but addition of a portion of the combustion air is delayed to yield oxygen deficient conditions initially and then to facilitate combustion of CO and any residual fuel.
OFA systems rely on the momentum of the OFA jets to provide effective mixing with the flue gas stream. For a given OFA mass flow rate, penetration into the flue gas stream and the rate of mixing is controlled by the size and number of individual OFA jets and by their corresponding velocity. Higher velocities and small openings result in faster mixing rates, while larger openings lead to better penetration of the air into the flue gas stream. In practical combustion systems, the maximum OFA velocity which can be applied is typically limited by the pressure inventory available in the combustion air supply system, such that mixing rate and jet penetration cannot be controlled independently.
Current OFA systems can apply some passive or active methods for controlling near field mixing. In these systems, large-scale flow structures may be generated that significantly reduce mixing effectiveness near the injector outlet. This leads to the need for higher airflow velocities that may not be attainable due to pressure inventory limitations.
Additionally, there is one embodiment of the SNCR process in which SNCR reagent is injected together with the OFA (AOFA/SNCR). At the high gas temperatures (1700-2400° F.) and moderate to high CO concentrations (0.2-1.0 percent) typical of AOFA/SNCR applications, CO competes with active species that are critical to NOx reduction thermochemistry. This reduces the effectiveness of the AOFA/SNCR process.
Earlier applications of the AOFA/SNCR process circumvented the CO oxidation problem by injecting very large reagent droplets into the overfire air. The droplets were carried by the gas through the boiler, eventually releasing N-agent into an optimal temperature window well downstream of the overfire air injectors where low CO concentrations exist. Unfortunately, the design of large droplet systems is difficult due to long droplet residence times, a tortuous flow path with obstructions, and often a narrow N-agent release temperature window. The N-agent can also generate NOx if it is released at higher temperatures upstream of the optimum temperature window. Ammonia slip can become an issue if the N-agent is released at lower temperatures downstream of the optimum temperature window.